The design and appraisal of a mass spectrometer, based on the focusing properties of mutually perpendicular electric and magnetic fields, is described. This field geometry spatially separates ions as a linear function of mass along a focal plane. This allows the spectrometer to be used in applications requiring the simultaneous monitoring of many ion species of independently varying intensities, such as from transient or pulsed ion sources, a feature not associated with other spectrometers. This instrument incorporates a novel detection system comprising a channel multiplier plate which considerably enhances sensitivity and a 50 element slatted collector array which separately detects the individual ion signals in the chosen mass range 20-70 u. To date a detailed experimental appraisal in the scanned E field mode, in which the resolved ion current spectrum is swept across one of the collectors, has provided data on the detailed mass peak profiles, the resolving power of the analysing E*B field and the efficiency and dispersion of the detector system. The data provided by this investigation has allowed a detailed appraisal to be presented concerning the modifications required in order that the instrument can be operated in the constant E field mode in which the E field is fixed and the resolved ion current is distributed amongst the 50 collectors.